The 24 Max Video Participants is the number of different sites connected to the conference.
The 20 Max CP Per Call is the number of sites' video displayed in the video sent to each the sites.
From one of the Cisco MCU 5300 manuals, the available continuous presence layouts available show a maximum of 20 sites being displayed at a time, in a 5x4 configuration. With a maximum of 24 sites being connected at a time, the 4 least active/quiet would not be seen, but as one of them would speak, they would replace one of the other 20 in the display.
